The present invention concerns a spacer and a fuel assembly for a nuclear reactor, preferably for a nuclear light water reactor. The spacer is designed such that it forms a plurality of cells for holding parallel elongated elements which extend through the cells or for allowing these elements to pass through the cells. Each of said cells is defined by a plurality of side portions which together surrounds one of said elongated elements when such an element is arranged such that it extends through the spacer. The spacer is designed to be arranged in a nuclear reactor such that said cells, and thereby said elongated elements, extend in essentially a first direction in which a cooling medium is intended to flow. Each cell thus comprises a first end which is intended to be positioned upstream with regard to said first direction and a second end which is intended to be positioned downstream with regard to said first direction. For at least one of said cells it is the case that at least one of said side portions which define the cell comprises at least one flow-influencing member which is formed by a part of said side portion and which is folded out from the side portion. Said side portion from which said flow-influencing member is folded out comprises a first and a second edge surface which are connected to each other, which edge surfaces have different principal directions of extension which correspond to the directions of a first and a second edge of said flow-influencing member in a non folded out state of the flow-influencing member.
A spacer of this kind is known from U.S. Pat. No. 5,112,571. The purpose with said flow-influencing member is to influence the flow of the cooling medium which flows through the spacer in order to improve the cooling of the parallel elongated elements which consist of fuel rods. The above mentioned flow-influencing member will below also be called fin. The mentioned U.S. patent document shows that the cells have a plurality of fins which are folded out from the side portions of the cells. The document shows one embodiment where a cut which extends in the vertical direction has been done from the upper edge of the cell. A part of the cell has thereby been folded out. The side portion from which the fin has been folded out thus comprises only one edge surface which extends essentially vertically. The document also shows a second embodiment where the fin is positioned further down in the cell. The side portion thereby comprises an upper edge surface which extends essentially horizontally and a second edge surface which starts from the upper edge surface and extends essentially vertically. The folded out fins thus have an essentially triangular shape.
U.S. Pat. No. 5,440,599 shows folded out fins which are arranged on top of the spacer itself, i.e. above the cells themselves which hold the elongated elements in position.
SE-C2-510 816 shows spacers where fins have been folded out from the spacer cell after that one or two vertical cuts have been made from the upper edge from the spacer cell.
The Swedish patent application 9803177-6, which has been made public, shows spacers with cells of an open construction where fins of essentially triangular shape are arranged.
It should be noted that it exist nuclear reactors of different kinds, for example so-called boiling water reactors and so-called pressure water reactors. In such nuclear reactors nuclear fuel is usually arranged in fuel rods. A plurality of such fuel rods are usually arranged parallel to each other in a fuel assembly. In order to hold the fuel rods at determined distances from each other, a fuel assembly comprises a plurality of spacers. FIGS. 1, 2 and 3 show different kinds of known spacers. Such spacers comprise a plurality of cells 10 through which the fuel rods are intended to extend. The cells 10 are arranged parallel to each other. Usually the cells 10, and thus the fuel rods, extend in the vertical direction. The spacer, and each cell 10 in the spacer, thus has a first end 16 which is located at the bottom and a second end 18 which is located at the top. A cooling medium, usually water, is intended to flow upwards through the fuel assembly. A main flow direction which thus is parallel to the extension of the fuel rods and the cells 10 is marked with 14 in FIG. 1. FIG. 1 shows a kind of spacer with sleeve-shaped cells 10. In the shown case, the cells 10 are cornered in order to hold the fuel rods in position with a spring force. The spacer of the kind that is shown in FIG. 1 will below be called a spacer with sleeve-shaped cells 10. Such a kind of spacer may also have cells 10 of essentially circular shape. The sleeve-shaped cells 10 are preferably welded together in order to form a spacer.
FIG. 2 shows a kind of spacer where the cells 10 are formed as relatively open elements with support points and resilient members which are arranged to hold the fuel rods in position. This kind of spacer will below be called a cell-spacer. Between the cells 10 in those kinds of spacers which are shown in FIG. 1 and in FIG. 2 a number of flow channels 32 are formed through which a cooling medium may flow.
In FIG. 3 another kind of spacer is shown which below will be called an egg-crate spacer. Such a spacer comprises a plurality of cells 10 of essentially square shape. Since the cells 10 have a squared shape, no further flow channels 32 of the kind which have been described above are formed in this kind of spacer. However, spaces are of course formed between the fuel rods 19 also in the kind of spacer which is shown in FIG. 3, in which spaces a cooling medium may flow.
In order to achieve a good cooling of the fuel rods is important that the cooling medium is conducted to the fuel rods in an efficient manner. The above described documents show different kinds of fins which are designed to conduct a cooling medium towards the fuel rods.
A purpose of the present invention is to achieve a spacer which in a more efficient manner contributes to directing a cooling medium towards the fuel rods. A further purpose is to achieve such a spacer with a relatively simple construction. Still another purpose is that the flow-influencing members which form part of the spacer shall have a relatively large flow directing surface without the necessity to make these flow-influencing members very large.
These purposes are achieved by a spacer of the kind which is described initially above and which is characterised in that said first edge surface extends from said second end of the cell and has an extension in at least said first direction and in that said second edge surface extends from that end of said first edge surface that is positioned upstream and has an extension in at least a direction which is perpendicular to said first direction.
Spacers according to the present invention thus have the advantage that the flow-influencing member, i.e. the fin, may be fabricated relatively simply since it forms a folded out part of the cell. Since the second edge surface, which is located upstream, has an extension perpendicularly to the flow direction, the folded out fin gets a relatively large flow directing area. It has in flow experiments appeared that the fin which forms part of the spacer according to the present invention contributes to an essentially improved cooling of the fuel rods.
According to a second aspect of the invention, the above mentioned purposes are achieved also by a spacer of the kind which is described initially above and which is characterised in that said first edge surface extends from said first end of the cell and has an extension in at least said first direction and in that said second edge surface extends from that end of said first edge surface that is positioned downstream and has an extension in at least a direction that is perpendicular to said first direction. According to this aspect of the invention, the same advantages are achieved as with the above described spacer. The difference is thus that the fin according to this second aspect of the invention extends from the end of the cell that is located upstream.
According to a third aspect of the invention, the above mentioned purposes are achieved by a spacer of the kind which is described initially above and which is characterised in that said first edge surface does not extend to said first end of the cell and does not extend to said second end of the cell and has an extension in at least said first direction and in that said second edge surface extends from that end of said first edge surface that is positioned upstream and has an extension in at least a direction which is perpendicular to said first direction. Also according to this aspect of the invention, the above-mentioned advantages are achieved since the second edge surface has an extension in a direction that is perpendicular to the flow direction. All the aspects of the invention are thus based on the idea that said second edge surface has an extension in a direction that is perpendicular to the flow direction, whereby a relatively large flow directing surface is achieved in the fin. This leads to an improved cooling of the fuel rods. The third aspect of the invention has the advantage that the spacer cell has a particularly rigid construction since the fin neither reaches to the first nor to the second end of the cell.
According to an advantageous embodiment of the invention, the folding line along which said flow-influencing member is folded out from said side portion is essentially straight and forms an angle of between 10xc2x0 and 40xc2x0, preferably between 20xc2x0 and 30xc2x0, with said first direction. It has appeared that the fin hereby gets an advantageous inclination relative to the flow direction such that an optimal influence on the flow of the cooling medium is achieved.
According to another advantageous embodiment of the invention, said flow-influencing member is folded out between 40xc2x0 and 80xc2x0, preferably between 50xc2x0 and 70xc2x0, from said side portion. It has appeared that since the flow-influencing member is folded out in this manner, an advantageous vortex is created in the flow of the cooling medium, which leads to an improved cooling of the fuel rods.
According to still another advantageous embodiment of the invention, said second edge of the flow-influencing member forms, when the flow-influencing member is folded out, an angle of between 70xc2x0 and 110xc2x0 with said first direction. Since said second edge in this manner is arranged more or less perpendicularly to the flow direction, the flow-influencing member gets a relatively large flow directing surface.
According to a further embodiment of the invention, said flow-influencing member is designed and arranged such that the whole flow-influencing member is positioned between said first and second end of the cell. The flow-influencing member is thereby positioned protected in the spacer. This means that there is no risk that the flow-influencing member is damaged, for example when fuel rods are exchanged.
According to still another embodiment of the invention, said first edge surface has a principal extension in a direction which forms an angle of between xe2x88x9240xc2x0 and +40xc2x0 with said first direction and said second edge surface has a principal extension in a direction which forms an angle of between 70xc2x0 and 120xc2x0 with said first direction. With such angles it is achieved that the folded out flow-influencing member has a flow-technically advantageous shape.
According to still another embodiment of the invention, said first and second edges are straight. Such a spacer is relatively simple to produce.
According to another embodiment of the invention, the spacer is of the kind where there are flow channels between said cells, in which flow channels a cooling medium may flow, wherein said flow-influencing member is folded into one of said flow channels. The spacer may thereby be of the kind which have sleeve-shaped cells or of the kind which is called a cell-spacer. With the invention it is achieved that the cooling medium which flows through these flow channels with an efficient manner is directed to the fuel rods for cooling the same.
According to a further embodiment of the invention, said flow channel defines an axis located centrally in the flow channel, which axis has an extension in said first direction, wherein said flow-influencing member is designed and arranged such that it reaches in to a position which is located at a distance from that side portion from which the flow-influencing member is folded out which constitutes between 50% and 90% of the distance between said side portion and said central axis. It is thereby achieved that the flow-influencing member has an optimal size. The flow-influencing member does not cause a too large pressure drop in the fuel assembly at the same time as the cooling medium in an efficient manner is conducted towards the fuel rods.
According to a further embodiment of the invention, a plurality of said flow-influencing members are positioned in each of at least a plurality of said flow channels. Thereby an improved control of the flow of the cooling medium in the direction towards the fuel rods is achieved.
According to still another embodiment of the invention, the spacer is of the kind which consists of an essentially square pattern, where essentially each cell is defined by four side portions which form a cell having a square form, which spacer does not include any further flow channels between said cells, wherein said flow-influencing member is arranged in a side portion close to a corner in the cell in question and is folded out into the cell. This kind of spacer is thus what is called an egg-crate spacer. In this kind of spacer there is a certain distance between the corners in the cell and a fuel rod that extends through the cell in question. This space may be used for arranging flow-influencing members.
According to a further embodiment of the invention, a plurality of said flow-influencing members are positioned in each of at least a plurality of said cells. For example, close to each corner, four flow-influencing members may be arranged in four different cells. In this manner, each cell may comprise one folded-in flow-influencing member in each corner, i.e. there are four flow-influencing members in each cell.
According to still another embodiment of the invention, said flow-influencing member has such an extension that it does not reach to that space that is intended to be occupied by a fuel rod when the spacer is arranged in a fuel assembly. The flow-influencing members do thus not exercise any supporting function for the fuel rods. Hereby a larger freedom is achieved in designing the flow-influencing members.
The purpose of the invention is also achieved by a fuel assembly for a nuclear light water reactor, which includes at least one spacer according to any one of the preceding embodiments.